Fluid droplet ejectors have been mostly associated with the printing business. Nozzles of various kinds have been reported in many publications and are commercially available. These nozzles are typically used to allow the formation and control of small ink droplets that result in high quality printing on demand.
Typically, an ink printhead has apertures or nozzles from which ink droplets are expelled onto a print medium, and the ink is routed internally through the printhead. Conventional methods of ejecting inks onto the print medium include piezoelectric transducers and bubbles formed by heat pulses to force fluid out of the nozzles. In situations where a printhead includes multiple nozzles, if one desires to selectively expel ink droplets from a specific nozzle and not the other nozzles, conventional solutions known in the art, isolate the nozzles from each other by long narrow passages that damp pressure surges in the ink fluid provided to the nozzles from a common source. Heaters can also be located at each nozzle, for the purpose of reducing ink viscosity at a specific nozzle. Thus, when a droplet is to be ejected from a specific nozzle, the heater at that nozzle is activated to heat ink at the nozzle so that when a pressure pulse is applied to the ink fluid, the ink viscosity at the nozzle is reduced enough so that a droplet of ink will be expelled from the nozzle, while the higher viscosity of the (colder) ink at the other nozzles remains high enough to prevent ejection of ink droplets from those other nozzles.
In U.S. Pat. No. 6,712,455, it is reported that a printhead includes a common ink chamber or reservoir bounded on one side by a membrane having nozzle apertures. The membrane forms a print face of the printhead. Piezoelectric elements (piezos) are located on the membrane near the nozzles. The piezos flex segments of the membrane surrounding the nozzles to eject ink droplets from the nozzle apertures. Ribs are also provided on the membrane and define boundaries of the membrane segments corresponding to the nozzles. The ribs can isolate each nozzle from the other nozzles, in two ways. First, the ribs act as stiffeners so that when piezos attached to one membrane segment flex that membrane segment, the other membrane segments are not significantly flexed. Second, when the ribs are provided on an interior surface of the membrane, they deflect the pressure pulse in the ink fluid from a flexing membrane segment, upwards, away from adjacent membrane segments/nozzles.
Micromachined droplet ejectors have also been reported in U.S. Pat. Nos. 6,445,109 and 6,474,786. This type of droplet ejectors include a cylindrical reservoir closed at one end with an elastic membrane including at least one aperture. A bulk actuator at the other end for actuating the fluid for ejection through the aperture. The ejector array is a micromachined two-dimensional array droplet ejector. The ejector includes a two-dimensional array of elastic membranes having orifices closing the ends of cylindrical fluid reservoirs. The fluid in the ejectors is bulk actuated to set up pressure waves in the fluid which cause fluid to form a meniscus at each orifice. Selective actuation of the membranes ejects droplets. In an alternative mode of operation, the bulk pressure wave has sufficient amplitude to eject droplets while the individual membranes are actuated to selectively prevent ejection of droplets.
These conventional and micromachined print heads or fluid ejectors suffer from various disadvantages. First, they usually require a large interconnected reservoir to store the ink or fluid. The fluid can only be ejected when this reservoir is fully filled, which usually results in large waste because these are considered dead volume. Second, the print head or ejector array has many long, narrow passages for transmitting ink to a particular nozzle. Third, many of these print heads and fluid ejectors address the need to selectively eject fluid from one particular nozzle. Because of manufacturing differences, however, these devices are not suitable to uniformly eject fluid in pico-liter quantities.
In biochemistry or related applications, there is a need for fluid ejectors that can control the fluid ejection at pico-liter level reliably. The fluid ejector is also required to have small dead volume so that there is least waste of biochemical reagents. In addition, it needs to eject fluid droplets uniformly across all orifices without satellite drops.